The present invention relates to internal combustion engines; more particularly, to devices for controlling the variable actuation of intake valves in an internal combustion engine; and most particularly, to a variable valve actuation assembly for controllably actuating and deactuating a rocker assembly responsive to a triple-lobed cam in an internal combustion engine between high valve lift and low valve lift modes.
Internal combustion engines are well known. In an overhead valve engine, the valves may be actuated directly by camshafts disposed on the head itself, or the camshaft(s) may be disposed within the engine block and may actuate the valves via a valve train which may include valve lifters, pushrods, and rocker arms.
It is known that for a portion of the duty cycle of a typical multiple-cylinder engine, especially at times of low torque demand, valves may be opened to only a low lift position to conserve fuel; and that at times of high torque demand, the valves may be opened wider to a higher lift position to admit more fuel. It is known in the art to accomplish this by providing a special rocker assembly having a switching or latching pin which may be actuated and/or deactuated electromechanically. The rocker assembly includes both fixed peripheral low-lift cam followers that cause low lift of the valve when the pin is disengaged, and a pivotable central high lift cam follower that causes high lift of the valve when the latching pin is engaged into the high lift follower.
Various methods for actuating this type of latching pin are known. For example, see the disclosures of U.S. Pat. Nos. 5,619,958; 5,623,848; and 5,697,333. All of these methods employ individual solenoids, acting through bellcranks or similar structures, as part of an actuation system.
A significant problem for these devices is how to balance the physical size of the solenoid against the force required to actuate the mechanism. The solenoid desirably has rapid response, small size, sufficient stroke and pull-in force, low power requirement, and low sensitivity to voltage and temperature variations; whereas, large size, high pull-in force, and high power are typically required to energize prior art mechanisms.
One approach, disclosed in the above-referenced patents, is to reduce the solenoid force required by using the rotational motion of the rocker assembly inherent in its duty cycle to supply a portion of the actuating force. Typically, the motion of the rocker assembly permits the solenoid to xe2x80x9cpull inxe2x80x9d to a low air gap wherein high actuating forces can be generated. The solenoid essentially locks itself in the engaged position during a valve lift event (lift portion of the duty cycle), and some other compliant element in the device, such as a bellcrank, resiliently deflects as the rocker returns to the base circle portion of the cam at the conclusion of the lift event. Once the rocker reaches the base circle, the energy stored in the compliant element causes the locking pin to become engaged with the high-lift follower, shifting the rocker assembly to high-lift mode. This configuration requires the holding force of the solenoid in the actuated position to be greater than the force exerted against it by the compliant element; otherwise, the motion of the rocker assembly will overcome the solenoid and increase the magnetic air gap within the solenoid to a point at which the solenoid force becomes too small to actuate the pin, and the rocker then does not shift to high-lift mode.
Another prior art approach, disclosed in U.S. Pat. No. 5,623,897, decouples the force generated by the compliant element from the locking force of the solenoid. One end of the compliant element is xe2x80x9cgroundedxe2x80x9d to the cylinder head, and the solenoid moves the opposite end of the compliant element into a position wherein it may engage the rotational displacement of the rocker assembly. The solenoid simply has to hold the compliant element in that position; it is not required to resist the internal force carried by the compressed compliant element.
The prior art configurations as disclosed have several shortcomings.
First, several of the linkages are fixed with respect to the pivot point of the rocker assembly, which typically is the ball-head of a hydraulic lash adjuster (HLA) supporting the assembly. The vertical length of the HLA may vary in the normal course of operating, and thus the pivot point may also vary in the z (vertical) direction. Further, the vertical and horizontal (x,y) locations of the pivot point must vary inherently from engine to engine as a result of stack-up of manufacturing tolerances. The prior art disclosures do not address practical or self-compensating means for accommodating tolerances in the cylinder head and cam cover.
Second, mechanisms disclosed in the prior art typically employ rotating linkages which may add friction to the force required for actuation and thus increase the force requirements of the solenoid.
Third, none of the disclosed mechanisms, except that shown in U.S. Pat. No. 5,623,897, fully decouples the solenoid force from the compliant element and, therefore, from the pin actuating force. In the disclosure of U.S. Pat. No. 5,623,897, a rotating rocker assembly with a large rocker ratio and large rotational inertia pivots through a relatively large angle in actuating the engine valve. These characteristics add to the force requirements of the solenoid. Further, the solenoid plunger does not act orthogonally to the rocker assembly, resulting in side-loading and friction in the solenoid bearings.
Fourth, in some prior art mechanisms, the point in the rotational cycle of the cam at which the solenoid is energized must be very carefully timed to avoid a phenomenon known in the art as xe2x80x9cejectionxe2x80x9d wherein the mechanism attempts to engage or disengage the locking pin into or out of the high-lift follower. When the pin is only slightly engaged, it is violently ejected, which can damage the pin or the high-lift follower and which causes a very loud and objectionable noise. Accurate timing of the solenoid energizing can be complex, as the response time of the mechanism may be affected by various operating parameters, such as oil temperature and thus viscosity.
It is a principal object of the present invention to provide an improved variable valve actuation (VVA) assembly wherein a secondary latching mechanism between the solenoid and the primary latching pin in the rocker assembly automatically self-times the engagement of the secondary latching mechanism such that the timing of solenoid energizing and de-energizing is not critical and ejections are prevented.
It is a further object of the invention to provide an improved VVA requiring a low solenoid actuating force and short stroke.
It is a still further object of the invention to provide an improved VVA wherein variation in assembly performance from the stack-up of manufacturing and operating tolerances among the components of the assembly is minimized.
Briefly described, a variable valve actuation assembly for variably opening of an engine intake valve in either a low-lift or high-lift mode includes a special rocker assembly pivotably disposed in the engine for opening and closing the valve and having a central high-lift cam follower and two peripheral low-lift cam followers, responsive to rotation of a camshaft having low-lift and high-lift lobes engageable with the respective cam followers; a primary latching mechanism including a slidable primary latching pin in the rocker assembly for engaging and disengaging the high-lift follower; a solenoid for causing the primary latching pin to be engaged and disengaged; and a secondary latching mechanism between the solenoid and the primary latching pin to automatically limit engagement and disengagement of the primary latching pin to times in the duty cycle of the camshaft when ejections are not possible.